In a semiconductor manufacturing process, there is often performed a film forming process in which a SiN (silicon nitride) film is formed on a substrate so as to cover a base film having a pattern such a hole, a groove or the like. In the related art, a film forming apparatus for performing formation of a SiN film in this way is known. According to this film forming apparatus, as a rotary table is rotated, a substrate mounted on the rotary table passes alternately and repeatedly through a region where a raw material gas containing silicon is supplied and a region where a nitriding gas for nitriding the raw material gas is plasma-converted and supplied. Thus, a SiN film is formed on the substrate.
There may be a case where, for example, a carbon film mainly composed of carbon, which is called a SOC film, is used as the base film mentioned above. However, if the base film is a carbon film, when an ammonia (NH3) gas is used as the nitriding gas and a process is performed by plasma-converting the NH3 gas, the carbon film is relatively heavily etched. Thus, film loss (film thickness reduction) occurs. In order to prevent such etching, it is conceivable to perform a process using a plasma-converted nitrogen (N2) gas instead of a plasma-converted NH3 gas. However, active species obtained by plasma-converting an N2 gas are relatively rapidly deactivated. For that reason, there is a possibility that most of the active species fail to reach the lower portions of a hole and a groove of a carbon film. In this case, in the SiN film formed on the sidewall of the carbon film which forms a pattern, the film thickness at the lower side of the sidewall is smaller than the film thickness at the upper side of the sidewall. Thus, when forming the SiN film on the carbon film in which a pattern is formed, it is difficult to obtain good step coverage.